Some earth-orbiting satellites contain optical imaging systems for tracking earth-bound targets. Generally, these systems generate real images that are converted into electrical signals and transmitted to the ground.
Depending upon the satellite's angle of inclination in relationship to the earth, the distance between the satellite and an earthbound target will vary. As a result, the image size of fixed sized objects will also change. A similar effect arises in scanning telescopic systems in aircraft. Here, as the telescope scans the terrain over which the aircraft is passing, depending on the telescope's angle inclination, the changing distance between the telescope and the various targets affect the relative image sizes.
The solution to maintaining image size independent of the distance between the satellite and the target is to use a zoom optical system that can increase magnification in response to the satellite's angle of inclination relative to the earth. One problem that arises, however, is the fact that traditional zoom optical systems have utilized refractive optical elements. Optical glass is the preferred material for refractive lenses, but this is restricted in wavelength from 500 to 950 nm. A few materials are available for wavelengths outside of the range but these have severe limitations in size and durability. One of the major defects of refractive optics is chromatic aberration which becomes more serious as the wavelength band increases. If this is a problem for a particular application, one has to use several lenses made of different materials which increases the weight of the optics. The weight of refractive optics is a problem in any event because refractive lenses are not lightweight as in the case of mirrors. Refractive optics are also much more susceptible to thermal effects than reflective optics, and controlling to thermal environment in space or airborne systems is not an easy task. A final problem in refractive optics relative to reflective optics is that they are subject to the deleterious effects of radiation because the light has to go through the material of which the lens is made, and the optical characteristic of materials are more sensitive to radiation effects than their physical characteristics.